You can only compress data to some extent. You could see if any of the files are encoded in an older or less efficient codec than what you have. Transcoding does introduce some distortion, but usually not too bad for utility use like learning material.

The other option is to get rid of some data. Delete some of the files. Use a (non-recompressing) editor to clip out parts you don't need. Transcode with lower resolution and/or bitrate/complexity.

It all depends on what you have and what you're able and willing to do with it. There is no universal silver bullet to get you more storage space. Other than actually getting more storage space.